This invention relates to cast materials for outdoor use. More specifically, it relates to a high strength, low expansion gypsum cement.
Casting of articles is a well known technique for making outdoor lawn and garden products, such as statuary, stepping stones and the like. The casting process, which includes preparing a mold, pouring a slurry of hydraulic material into the mold and allowing it to harden, is less labor intensive, and therefore less expensive, than many other methods of making a shaped article.
Both gypsum and cement are well known hydraulic materials. Gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Plaster of Paris is also known as calcined gypsum, stucco, calcium sulfate semihydrate, calcium sulfate half-hydrate or calcium sulfate hemihydrate. In this form, there are approximately two water molecules of water associated with each molecule of calcium sulfate. In order to produce the hemihydrate form, the gypsum can be calcined to drive off some of the water of hydration by the following equation:CaSO4.2H2O→CaSO4·1/2H2O+3/2H2O
Calcium sulfate hemihydrate can produce at least two crystal forms. α-Calcined gypsum is made by a continuous process or a lump rock process whereby the calcium sulfate dihydrate is calcined under pressure. The α-calcined gypsum forms less acicular crystals than β-calcined gypsum, allowing the crystals to pack tightly together, making a denser and stronger plaster. The crystal morphology allows water to flow easily between the crystals, requiring less water to form a flowable slurry. More elongated crystals are characteristic of the beta-calcined gypsum. This crystal structure results in a less dense product because the crystals are more loosely packed. The beta form also requires more water to fluidize the calcined gypsum. If the calcining of the dihydrate is performed at ambient pressure, the beta form is obtained and the cost is relatively low compared to the alpha-gypsum.
When the hemihydrate is added to water, the product slurry is permitted to set by allowing the calcium sulfate hemihydrate to react with sufficient water to convert the hemihydrate into a matrix of interlocking dihydrate crystals. As the matrix forms, the product slurry becomes firm and holds the desired shape.
Hydraulic cement is used in various applications where its hardness, water resistance and durability make it valuable, such as in concrete structures. Products that require wearability, hardness or water resistance are often made of a slurry of calcined gypsum mixed with hydraulic cement. Cements, such as Portland cement, set by the relatively slower hydration reactions of calcium silicate and aluminate materials. Consequently, adding calcium sulfate hemihydrate to cement offers a faster set and the resulting increase in productivity during manufacture cement-containing products. Gypsum is, however, somewhat soluble in water, and mixtures that contain both gypsum and hydrated cement are not as water resistant as cement alone or cement containing a minor amount of gypsum.
Cast articles for outdoor use should be strong to hold up to wind, weather and other outdoor hazards. When no additives are used, the amount of water added to a calcined gypsum slurry determines the density of the set gypsum matrix. As more water is used, the slurry increases in volume. The theoretical water demand of pure calcium sulfate hemihydrate is 18.6 wt %. The hydrated calcium sulfate matrix forms filling the volume originally occupied by the slurry, trapping the excess water in the crystal interstices of the gypsum matrix. For equal amounts of gypsum, the interstices are larger and more numerous to take up the excess water as the proportion of water is increased. As the size and number of the spaces increases, both the density and the strength of the matrix decreases compared to a composition with little of no excess water. However, slurries with little excess water are very difficult to mix, particularly if beta-calcined gypsum is used.
However, if water is reduced to increase the strength of cast articles of this type, there is generally an increase in the expansion of the article as the hydraulic materials set. Expansion of hydraulic materials while they set limits the useful life of the molds with which they are used. The expanding article puts pressure on the mold, forming small stress cracks which grow bigger with successive uses. Details in the mold can be lost. The article can become distorted as the mold weakens.
Thus, there is a need in the art for a means to improve the strength of cast articles while creating minimum expansion of the setting slurry material. There is a further need to maintain dimensional stability of cast articles, while extending the life of the molds from which they are made. When these needs are met, products made by casting gypsum or blends of gypsum and hydrated cement can be made stronger, maintain design details and/or be produced at lower cost.